Birds use acoustic signals (songs) in mate attraction and intrasexual competition. Therefore, these vocalisations are influenced by natural and sexual selection, shaping species-specific songs. According to Bergmann’s rule, song frequency should vary with latitude and altitude due to its link with body size, which is constrained by temperature. This study examined the fiery-necked nightjar (Caprimulgus pectoralis) to assess how biogeographic gradients and subspecies differentiation affect song characteristics. Using recordings from across the species’ entire distribution range in Africa, we analysed peak frequency, rhythm (Inter-Onset Interval, IOI) and frequency–modulation ratio to test predictions related to Bergmann’s rule and subspecies divergence. Consistent with Bergmann’s rule, elevation significantly affected peak frequency, with lower song frequencies at higher elevations (i.e. in cooler habitats). In contrast, latitude had no significant effect on peak frequencies. Song rhythm differed between subspecies, with C. p. shelleyi exhibiting longer IOIs than C. p. pectoralis, suggesting potential ecological or behavioural adaptations. Our findings highlight the influence of environmental factors and subspecies differentiation on vocal traits and emphasise the complex interplay between ecology, physiology, and evolution in a nocturnal bird. We suggest that future studies should investigate additional environmental and behavioural factors to elucidate the adaptive significance of these patterns.
Animal communication, Bergmann’s rule, bird song, body size, Caprimulgus pectoralis